This patent specification is in the field of radiography and tomography for patients or inanimate objects, and in the field of x-ray bone densitometry.
Digital radiography has made significant inroads in recent years but it is estimated that more than half of the x-ray imaging procedures still involve film. One of the reasons may be that the cost of a two-dimensional digital detector of the same size as the x-ray film typically used in this country, e.g., 14 by 17 inches, is still quite high. Another reason may be that a flat panel digital x-ray detector of this size may receive scattered radiation that could interfere with quantitative work such as estimating bone density. A chest x-ray system using a two-dimensional flat panel x-ray detector is commercially available in this country from Direct Radiography Corporation of Newark, Del. a subsidiary of the assignee of this patent specification, Hologic, Inc. of Bedford, Mass. See, e.g., U.S. Pat. No. 5,166,524, hereby incorporated by reference.
In bone densitometry using penetrating radiation such as x-rays, one-dimensional arrays of detectors have been used, for example in the QDR series bone densitometers commercially available in this country from the assignee of this patent specification, Hologic, Inc. of Bedford, Mass. See, e.g., U.S. Pat. Nos. 5,838,765 and 5,850,836, which are hereby incorporated by reference. In addition, devices that use two-dimensional flat panel detectors for other purposes, such as for fluoroscopy, can be adapted for use in bone densitometry, as in the case of small C-arm x-ray fluoroscopy systems commercially available in this country from FluoroScan Imaging Products, Inc. of Northbrook, Ill., also a subsidiary of Hologic, Inc.
The use of a two-dimensional array of detector elements has been proposed in mammography, in a scanning system. See, e.g., U.S. Pat. No. 5,526,394. The use of two rows of low-energy and high energy-detectors has been proposed for x-ray bone densitometry and baggage inspection. See, e.g., U.S. Pat. No. 5,841,833.
Film tomography has been used for some time in this country, and typically involves moving an x-ray source on one side of a stationary patient and film at the other side in opposite directions while the patient remains stationary. The source typically emits a cone-shaped or pyramid-shaped x-ray beam. In the course of imaging, a planar region in the patient that is often called the focal plane and is parallel to the film, is in better focus in the image while the rest of the patient""s body is more blurred. The image is not very clear, but can still be diagnostically useful in some cases.
It is believed that a need remains for a system using a two-dimensional detector in a manner that would be more effective than the known prior art proposals, and certain aspects of the system and method disclosed herein is directed to meeting such a need. Further, it is believed that improvements in tomography are desirable, and certain aspects of the system and method disclosed herein are directed to meeting such a need. While the disclosure below is mainly in the context of patient examination, the system can be used to examine inanimate object as well, such as in baggage or product inspection.
Certain embodiments of the x-ray system disclosed in this patent specification use a two-dimensional, slot-shaped detector of x-rays, e.g., a 25xc3x971 cm detector that has pixel size of approximately 100 micrometers and is scanned along the image plane but also can rotate about the focal point of the x-ray source and can move along the x-ray propagation direction relative to the patient. The pixel values can be processed to obtain tomosynthetic information and for other purposes as well. The focal plane or focal surface that is in best focus is a function of the geometry and other characteristics of the relative motion between at least two of the x-ray source, the patient, and the detector and the number of pixel rows that are shifted before adding pixel values from different time frames. For example, if the detector has n rows and m columns of pixels, and both the x-ray source and the detector scan the patient in a linear motion, one planar or non-planar region (focal plane) in the patient can be kept in effective focus by shifting and adding rows of pixel values. If the system adds the pixel values for pixel row n and time frame 1 to the pixel values for pixel row n+1 and time frame 2, (or row n+2, n+3, . . . ), etc., a tomosynthetic image can be formed that can correspond in principle to the information content of a conventional, film tomographic image obtained by moving the x-ray source and film cassette in opposite directions. If the system disclosed herein uses a pulsed x-ray source, the pulse duration t should be short compared to the speed c of translation of the source/detector such that ct less than (pixel size), or at least ct less than (desired spatial resolution). Similar consideration applies if the system uses a constant x-ray source, in which case pixel integration (interrogation) time is the parameter of interest. If the system uses a dual-energy pulsed x-ray source or beam, this must be taken into account in any addition of pixel values from different time frames. For example, if the energies alternate in the x-ray pulse train, pixel values from odd frames can be added together for image information for one energy, and pixel values from even frames can be added together for image information pertaining to the other energy. As an alternative to a pulsed dual-energy x-ray source or beam, low-energy detector elements and high-energy detector elements can be used to concurrently measure the high and low x-ray energy, as known in the art.
When the disclosed system is used for a tomosynthetic image, the plane or surface that is in best focus is defined by the geometric formula b=sw/d, where s=distance out of the plane, w=width of the x-ray detector, and d=distance between the x-ray source and detector. Other planes or surfaces in the patient are blurred, which can be desirable as it in effect makes transparent tissue that is away from the plane or surface in focus, but can be undesirable if the tissue of interest extends along the length of the x-ray beam or if the tissue of interest curves out of the plane or surface of interest (as the spine of a person lying on his/her side). It should be understood that in practice the xe2x80x9cplanexe2x80x9d or xe2x80x9csurfacexe2x80x9d in focus has some thickness in the direction of x-ray propagation, i.e., there is some depth of focus as in conventional photographic exposures. One way to deal with depth of focus issues is to store all pixel values, or at least all that could pertain to the volume or interest, so that any tomosynthetic image can be generated from them as desired so long as it is within the volume for which pixel values were stored, and the user can command the generation of images of different planes when looking for tissue of interest. Significant data storage requirements need to be taken into account in this approach. Another solution is to dynamically track the location of the feature of interest, by varying the depth of the plane or surface that would be in best focus. If the feature is the spine of a patient, such dynamic tracking can select the shift-and-add parameters discussed above so as to keep the spine in best focus, e.g., to make the gaps between the vertebrae the sharpest, or perhaps the pedicles.
Additionally, rotating the x-ray detector about the x-ray source 30 can produce pixel values for a non-tomosynthetic image that can be reconstructed with all projected planes in focus. The system disclosed herein can operate in both modesxe2x80x94with the x-ray detector rotating about the focal spot in the x-ray source (e.g., for high resolution), and with the x-ray source and the x-ray detector moving, for longer scans. In addition, the detector can be moved further away from the patient when desired to further reduce the effect of x-ray scatter, for example for more accurate bone density estimates.
Other aspects of the disclosure herein relate to obtaining tomographic images using a slit-shaped post-patient collimator in a process in which at least two of the x-ray source, the patient, and the imager move relative to each other. The imager can be an x-ray film cassette, or a digital flat panel detector that can be the same size as conventional x-ray film, or a slot detector such as discussed above. When the imager is a film cassette or a larger area flat panel detector, and the source emits a primary beam collimated to a fan-shape by a source-side aperture, the post-patient collimator can be shaped and dimensioned to allow only the primary, fan-shape beam to reach the imager. This collimator can be in the form of a slot in an x-ray opaque material such as lead of suitable thickness to effectively block radiation impinging thereon outside the slot therein. When the imager is a slot-shaped detector, such as the 25xc3x971 cm detector earlier mentioned, the post-patient collimator can be omitted, although it can still be used to absorb some scattered radiation. It is important to note that the post-patient collimator need not have the type of septa conventionally used in Bucky grids, i.e., it can be simply an open slit between two lead plates, or can be a material easily penetrated by x-rays at the relevant energy range. Thus, while a Bucky grid used in conventional radiography can significantly reduce the amount of radiation that would otherwise reach the imager, the post-patient collimator can allow nearly all of the radiation in the primary beam emerging from the patient to reach the imager.
When taking a tomographic image, exposure can be controlled through a feedback loop to avoid overexposure or underexposure at least along a selected path along the image, for example at the image of the patient""s spine. This can overcome a problem particularly evident in lateral lumbar/thoracic images, where the lumbar region can require ten to twenty times the x-ray dose of the thoracic region. In a preferred embodiment, this is done in real time, by monitoring the exposure behind the imager during the scan and adjusting the relevant x-ray beam characteristics accordingly. This can involve adjusting one or more of: (1) the current (mA) of the x-ray tube; (2) the voltage (kVp) of the tube; (3) the power (mAs), which can be done by reducing/increasing the duty cycle of the tube, e.g., by turning the x-ray tube off and on rapidly, by changing the scan speed, and/or by changing the beam aperture at the source side that determines the dimensions of the primary x-ray beam; and (4) the filtration through which the x-ray beam passes before impinging on the patient, e.g., by inserting/removing filters or by sliding or rotating or otherwise moving a filter to change the beam attenuation it causes.